1. Field of the Invention
The present invention relates to an air classifier that can effectively produce toner powder and other powder materials with sharp particle size distribution by preventing contamination with fine powder and coarse particles.
2. Description of the Related Art
Several traditional approaches are known for classifying (or sorting) pulverized coarse toner particles: a combination of a single classifier BZ1 and a single pulverizer FZ1 as shown in FIG. 1; a combination of two classifiers BZ1 and BZ2 and a single pulverizer FZ1 as shown in FIG. 2; and a combination of two classifiers BZ1 and BZ2 and two pulverizers FZ1 and FZ2 as shown in FIG. 3. One type of the pulverizers used in these systems is a jet pulverizer that propels raw material particles in a high pressure air stream spouted from a jet nozzle to cause the particles to collide with each other or hit a wall or other objects and thus crush (or pulverize) the particles. One example of such pulverizers is I-type mill pulverizer manufactured by Nippon Pneumatic Mfg. Co., Ltd.
An exemplary system is now described with reference to FIG. 3.
Raw materials are fed through a feed pipe FE1, and together with a previously pulverized product and high pressure air, introduced into a first classifier BZ1 where they are classified into a coarse powder and a fine powder. The coarse powder is pulverized in a first pulverizer FZ1 via a pulverizing unit and collected in a cyclone CY1. The collected powder is introduced into a second classifier BZ2 where it is again classified into a coarse powder and a fine powder. The separated coarse powder is then pulverized in a second pulverizer FZ2 via a pulverizing unit and collected in a cyclone CY2. The collected powder is sent to a fine powder-classifying unit where it is classified into a fine powder and a final product. In this system, however, the powder fed to the classifying unit contains toner particles of various sizes that are in the process of pulverization and are circulating between the pulverizing unit and classifying unit, as well as the raw material powder.
In FIGS. 1 though 3, BF1 and BF2 each indicate a bag filter, BL1 and BL2 each indicate a blower, and FE2 indicates a feed pipe.
FIG. 4 shows a construction of an air classifier (a DS air classifier) that is used as BZ1 and BZ2 in the above-described system. The air classifier includes a dispersion chamber (or collector dispersion chamber) 1, a classification chamber 2 and a bottom hopper 3 that are arranged from the top down. The dispersion chamber 1 is defined by a cylindrical casing 10 that has a dispersion chamber inlet 1a connected at the upper periphery thereof for feeding a primary air stream and powder materials to the dispersion chamber 1. Arranged within the dispersion chamber 1 near its bottom is an umbrella-shaped center core 5 with a raised center portion. A similarly umbrella-shaped separator core 8 with a raised center portion is arranged below the center core 5. A slatted secondary air stream inlet 9 (also referred to as “louver”) is arranged about the classification chamber 2 along the outer periphery of the classification chamber 2 to facilitate dispersion of the powder materials and accelerate the swirling of the powder materials. In this manner, the fine powder within the classification chamber 2 is guided to a fine powder discharge port 7 provided in the separator core 8 and discharged through a pipe 13 connected to the fine powder discharge port 7 by the suction force provided by the blower. On the other hand, the coarse powder is discharged from an annular discharge port 6 provided along the outer periphery of the lower edge of the separator core 8.
A typical DS air classifier operates by the principle that centrifugal and centripetal forces of different magnitudes act on the coarse particles and fine particles present in a powder material as the secondary air stream flows into the classification chamber and causes a non-free flow of the swirling particles. For this reason, it is desirable that the particles dispersed in the classification chamber be quickly classified into coarse particles and fine particles without allowing the particles to re-aggregate together.
However, conventional DS air classifiers are now required to disperse an increased number of toner particles because toner particles are becoming increasingly small and pulverization performance of pulverizers has improved significantly. When used to disperse such increased number of particles, the dispersion performance of conventional DS air classifiers will decrease, resulting in decreased classification accuracy. This inevitably leads to an increase in the amount of ultra-fine powder caused by excessive pulverization and coarse particles contaminating the fine powder discharge unit. As a result, the product obtained by the classification process may cause smears and improper transfer and may therefore lead to decreased image quality. The increased amount of ultra-fine powder and the contamination of the fine powder discharge unit with coarse particles may also pose an excessive load on the classifier during the production process and may thus decrease the efficiency of classification as well as the energy efficiency of pulverization.
Japanese Patent No. 2766790 discloses a classifier in which a louver is provided in the dispersion chamber (collector). In this classifier, a nozzle is inserted in the louver for introducing powder and primary air. Secondary air is introduced from the outer periphery of the louver to facilitate the dispersion of the powder. This construction is disadvantageous in that when raw materials are fed with high pressure air, the pressure difference within the dispersion chamber causes the raw materials to be released from the collector into the atmosphere, making it difficult to further continue the classification process.